RU2505740C2 - Method for production, storage and decomposition of natural gas hydrates - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: method includes preliminary filling of a ship by ice-water mixture. Natural gas hydrates (NGH) are produced at the ship by pressure pumping of natural gas to it and further gas bubbling through ice-water mixture with simultaneous pumping of the above mixture to it and vibrating at acoustic frequency until NGH fill the whole ship volume. Thereafter pumping of gas and cryohydric mixture is stopped; NGH are stored at the ship at permanent temperature and pressure. In order to decompose NGH into water and gas pressure is decreased in the upper part of the ship by gas extraction from it and/or ship lifting to decrease hydrostatic pressure in it. When gas extraction is completed the ship is lowered to the initial depth and ice-water and cryohydric mixture is stored at the ship till next filling by NGH.

EFFECT: creating conditions for use of a submerged ship as a combined means for production, underwater storage and decomposition of NGH.

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Description

The invention relates to the gas industry and can be used in gas supply systems of coastal, preferably coastal settlements and industrial enterprises.

A known method for the production, transportation and storage of gas hydrates (RU No. 2200727). According to the known method for producing hydrates at the processing site, oil and water are separated from natural gas and its mixture with other hydrocarbons, after which the purified gas is subjected to compression and cooling. The condensed gas obtained by this compression and cooling is removed in a separator in which such temperature and pressure are set to obtain certain hydrocarbons, preferably butane and higher hydrocarbons. The isolated chilled gas is then subjected to compression, cooling and fed into the reaction vessel and together with water, where the water and gas react to form a natural gas hydrate, consisting of frozen water with gas incorporated therein. The hydrate formed as a fine powder is discharged from the reactor. The hydrate powder is then cooled, pressed to obtain a higher density natural gas hydrate and used for gas storage and transportation. Particles of gas hydrate, according to the described method, can be stored on offshore platforms in underwater vessels under pressure. These vessels can be located on the seabed or near the platform. The hydrate particles can be stored as a solid in a gas or in chilled water or a hydrocarbon liquid. In addition to underwater vessels, tankers, barges, etc. can be used. or underwater tanks made of rigid or flexible materials.

The disadvantages of this method are:

- the need for a separate reaction vessel to obtain gas hydrates;

- difficulties with the processing of natural gas, which mainly consists of methane, due to more stringent conditions for the formation of stable hydrates from methane, compared with higher hydrocarbons;

- the need to transport hydrate powder in a "metastable" state from the reaction vessel to a container designed for long-term storage.

Closest to the alleged invention is a method of transporting natural gas in a ship immersed in water (Norwegian patent No. 149976). According to the known method "... natural gas and raw water are fed in separate streams into a ship submerged under water, where they form a gas hydrate, which is then maintained in a stable state due to hydrostatic pressure and the relatively low temperature of sea water ...".

The disadvantages of this method are:

- rapid heating of the mixture of water and gas in the vessel due to the high heat during the formation of hydrate (approximately 500 kJ per 1 kg of hydrates); as a result, each kilogram of water supplied to the vessel, from which approximately 1.13 kg of natural gas hydrates is formed, for example, is theoretically able to heat 1 kg (liter) of water by 120 ° C, which immediately leads to an unacceptable increase in the temperature of the water inside the vessel and terminates further hydrate formation in a vessel, for the continuation of which it will be necessary to either sharply increase the pressure in the vessel's hull to hundreds of kg / cm and / or cool the heated water to the initial temperature of sea water, for example, 0 ° C; obtaining high pressures in the vessel and lowering the water temperature take time, the use of special equipment and high energy costs;

- the supply of gas and raw water to a submerged under water (hereinafter referred to as the underwater) vessel does not provide high efficiency and the rate of formation of gas hydrates due to the gravitational separation of gas and water inside the vessel and the occurrence of a horizontal contact surface between gas and water, which will hydrate formation occurs, the area of which is limited by the size of the vessel, is uncontrollable and insufficient for the commercial rate of hydrate production; it was experimentally established that natural gas hydrates at equilibrium temperatures and pressures do not form inside large volumes of gas-saturated water, but form only on the contact surfaces of gas and water;

- taking into account the above drawbacks, it is practically impossible to periodically reuse the vessel as an apparatus for receiving, storing and subsequent decomposition (regasification) of natural gas hydrates, i.e. as an underwater gas storage, similar in purpose to the well-known underground gas storage, for example, in caverns in rock salt deposits.

The aim of the invention is to remedy these shortcomings for the use of a submerged ship as a combined device for receiving, underwater storage and decomposition of natural gas hydrates in gas supply systems of coastal settlements located on the shores of the northern seas, with an annual cycle of gas filling converted inside the ship into natural gas hydrates during the summer navigation, storage and selection of gas for supply to the consumer during the inter-navigation cold season a.

This goal is achieved by the fact that

- the submarine is filled in advance (as a cold accumulator, which maintains a constant melting temperature until the ice is completely thawed) with a cryohydrate mixture of the most compacted finely ground or granular, preferably salted sea ice and sea water, which preserves open porosity, for example, 20% by volume;

- when receiving hydrates, the cryohydrate mixture in the vessel is subjected to vibration of acoustic frequency, for example, 60 Hz, using submersible vibrators installed uniformly and approximately in the center of the volume of the submarine vessel, which provides additional crushing of bubbling natural gas bubbles, increasing the surface area of gas and water contacts, and also contributes to the compaction of the resulting hydrate;

- natural gas is pumped into the submarine from under the lower plane of its volume, at a pressure of at least 10 ati, usually liquefied natural gas (LNG), having a temperature of about minus 160 ° C, evenly distributing it over the entire cross-sectional horizontal section vessels released from gas distribution devices in the form of gas bubbles with a diameter of, for example, 0.005 m, which bubble in the vessel through the water in the pores of the ice-water mixture from the bottom up, almost instantly forming hydrate crystals at their borders with water, decreasing due to this, in size until complete disappearance;

- gas is injected into the vessel simultaneously with the injection of an ice-water mixture until the hydrates floating in the water fill the entire volume of the vessel and a significant increase in temperature inside the vessel begins, the accumulation of natural gas in its upper part and floating up that do not fit in it hydrate crystals in the water around the vessel, their decomposition into gas and water and the appearance of bubbles and jets of gas (griffins) on the surface of the water around the vessel;

- after this, the cryohydrate mixture is stopped and the obtained natural gas hydrates are stored in the vessel for the required time, for example, six months, at a constant temperature and pressure, if necessary, periodically pumping some additional ice-water mixture into the vessel in order to compensate for the external heat influx into the vessel and maintain it in it the necessary constant temperature;

- to decompose the hydrates stored in the vessel into water and gas supplied to the consumer (after drying and, if necessary, increase the pressure), lower the pressure in the upper part of the vessel, for example, to 5 atm, by taking (discharging) gas from the vessel into the gas pipeline to supply it to the consumer, and also raise the vessel up to reduce hydrostatic pressure in it, while the heat necessary for the decomposition of hydrates is released when the sea water freezes from the bottom of the vessel due to hydrostatic pressure, as well as capillary pulling of water up the pores of the cryohydrate mixture obtained and stored in the vessel;

- at the end of the full or partial selection of gas from the vessel to the consumer, the vessel is lowered to the initial initial depth, the water-ice and cryo-hydrate mixture formed after hydrate decomposition is stored until the vessel is replenished with gas hydrates, for example, for six months, periodically replenishing its amount (if necessary) with ice-water injections mixtures to compensate for external heat influx from the surrounding sea water.

The drawing shows a schematic diagram of an embodiment of the invention.

The vessel 1, communicating in the lower part with the surrounding water 2, is immersed in it upwards with a keel to a depth of about 100 m, suspended, for example, on cables 3 to pontoons 4 floating on the surface of the water. Vessel 1 is equipped with pipelines: 5 - for loading it with an ice-water mixture from floating 15 or above-ground 17 refrigeration unit; 6 - for supplying to a distributor (dispersant) 7 through a distributed system of perforated tubes located under the lower surface of an ice-water mixture 8 of natural gas loaded into the vessel 1 from the surface of the water area from an LNG tanker 9 or a supply gas pipeline 10; pipeline 11 for the selection of gas from the upper part of the vessel 1 resulting from the decomposition of the hydrate (after storage for the required time) into gas and water (ice), for supply to the consumer. In addition, in the center of the vessel 1, vibrators 12 are installed with an input electric cable 13, for example, vibration electric pumps, as well as a temperature sensor 14 inside the vessel 1. Pipelines 10 and 11, as well as an electric cable 13 for supplying the vibrators 12 and the information cable of the temperature sensor 14 are connected gas distribution control point 16.

The proposed method is as follows. On the surface, a mixture of preferably natural crushed sea ice (or prepared artificial, for example, granulated) with sea water is prepared and, for example, it is pumped in the form of ice-water pulp by pump through pipeline 5 to the lower part of vessel 1, in which ice floats up and gradually from top to bottom fills the hull of the vessel 1 to the gas distributor 7. Then, through the pipeline 6, they are pumped into the distributor 7 under pressure, for example, 12 ati, natural gas from the gas pipeline 10 or, preferably, from the LNG tanker 9 in liquid ide, with a flow rate corresponding to the complete conversion of the supplied gas to hydrates when it passes through bubbles displaced from the distributor 7 into the mass of the thickness of the ice-water mixture in the vessel 8 and sparges through it from the bottom up. The process of hydrate formation is carried out at a constant pressure in the vessel and at a constant melting temperature of sea ice, for example, minus 2 ° C, supported by a melting mixture of water and ice until almost all of the ice inside the vessel melts, and during this period, if necessary, increase the total of the resulting hydrates, an ice-water mixture may be supplied via line 5 in addition to its previously supplied amount. The supply of gas in the form of LNG allows you to add to the vessel 1 some additional amount of cold necessary for the formation of hydrates of natural gas, and thereby reduce the required amount of ice-water mixture. Throughout this period, the ice-water mixture 8 inside the vessel 1 is subjected to continuous vibration of the acoustic frequency, for example, 60 Hz with a total power, for example, 2 kW, of electromechanical vibrators 12, fed through cable 13, thereby accelerating the process of hydrate formation from water and gas. The temperature of the ice-water mixture 14 determines the beginning of a significant increase in the initial temperature in the vessel, which, along with the accumulation of free gas in the upper part of the vessel and gas bubbles on the surface of the water around the vessel, indicates the filling of the vessel with hydrates, after which the gas supply through the pipeline 6 is stopped.

Gas hydrates in the form of crystals gradually replace ice and also float to the upper part of the vessel 1, forming a dense but porous mass floating on the water, as a result of which a certain volume of hydrate crystals will remain in the dome part of the vessel 1, the pores between which are filled with gas. This space can always be created by increasing the flow rate of gas supplied through the pipeline 6, which will cause a slight increase in pressure in the pipeline 11.

After the formation of hydrates in the vessel 1, they are stored at constant temperature and pressure for the required time, using the vessel as an underwater storage of natural gas in a hydrated state.

If it is necessary to decompose hydrates into gas and water for supplying gas to the consumer through pipeline 11 in the pipeline, they simply lower the pressure by releasing gas into the consumer’s network and / or lift the vessel up on the pontoons with 4 ropes 3, thereby reducing hydrostatic pressure to provide the necessary hydrate decomposition rate in the vessel 1 Since the actual pressure will be lower in the upper part of the vessel, hydrates at the same temperature will decompose faster at the top, and sea water will lift the remaining hydrates up and fill the pores. forward hydrate crystals. During the decomposition of hydrates, the absorbed heat leads to the conversion of all the water contained in the hydrates into ice crystals, and the additional amount of heat is absorbed by freezing the water entering the vessel and the formation of an additional quantity of ice crystals, which will fill its entire volume by the end of gas extraction from vessel 1. In this state, the vessel 1 filled with ice-cold cryo-mixture is lowered (if it was raised) to its original position and stored at constant temperature and pressure until it is replenished with gas and the gas-storage-storage cycle is repeated completely. At the same time, during storage, to compensate for external heat influx into the vessel 1 from the water 8, an additional insignificant amount of the ice-water mixture can be periodically supplied to it through the pipeline 5. Vessel 1 can be either leaky (for example, a regular ship turned upside down) or hermetic, for example, a submarine or an ice tank, which must be equipped with a controlled valve for communication with sea water. Submarines equipped with autonomous ascent and diving facilities can be used without pendants.

Claims (1)

The method of obtaining, storage and decomposition of natural gas hydrates, including the supply of water and gas to a submerged vessel, the formation of natural gas hydrates from them and maintaining them in a stable state due to hydrostatic pressure and the relatively low temperature of sea water, characterized in that, the purpose of increasing the speed and efficiency of hydrate formation and ensuring the cyclical process of formation, long-term storage and subsequent decomposition of hydrates into gas and water (ice), a ship submerged under water before The cryohydrate mixture is preferably filled with finely ground preferably sea or artificially salted ice with preferably sea salted water, the volume of which in the said vessel is, for example, at least 20% of the total volume of the cryohydrate mixture, after which, without stopping the injection of the cryohydrate mixture, they are pumped into the lower part of the vessel and natural gas, preferably liquefied, is uniformly dispersed over the horizontal sectional area of the vessel with bubbles with a diameter of, for example, 0.005 m at a constant ice melting temperature in s bottom and hydrostatic pressure exceeding the equilibrium pressure of hydrate formation of natural gas, while vibrating the resulting mixture with electromechanical vibrators, until the vessel is filled with hydrates, and then the pumping of the ice-water mixture into the vessel is stopped; for hydrate decomposition, the vessel is raised up to obtain the required hydrate decomposition rate due to the reduction of hydrostatic pressure in the vessel significantly below the equilibrium pressure, the natural gas accumulated in the upper part of the vessel, which is released during the decomposition of hydrates in the entire volume of the vessel, is diverted to the consumer network; at the end of gas sampling, the vessel is lowered to the initial depth, and the ice-water mixture formed as a result of hydrate decomposition is stored in the vessel and, if necessary, replenished by pumping the same mixture from the surface until the cycle for receiving, storing and decomposing natural gas hydrates is repeated.

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